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Morphometric and ultrasonographic determinants of difficult laryngoscopy in obese patients: A prospective observational study

Wu, Yu-Minga,b; Su, Yen-Haoc,d; Huang, Shih-Yua,b; Wang, Chien-Wuna,b; Shen, Shih-Chiangc,d; Chen, Jui-Taia,b; Lo, Po-Hana,b; Cherng, Yih-Giuna,b; Wu, Hsiang-Linge,f; Tai, Ying-Hsuana,b,*

Author Information
Journal of the Chinese Medical Association: May 2022 - Volume 85 - Issue 5 - p 571-577
doi: 10.1097/JCMA.0000000000000721
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Obesity is a major epidemic of the twenty-first century.1 The global prevalence of overweight and obese individuals has doubled since 1980, and almost a third of the population worldwide is now classified as being overweight or obese.1 Obesity increases the risk of various types of diseases, including cardiovascular and metabolic disorders, liver and gallbladder diseases, gastroenterological cancers, and osteoarthritis.2,3 Accordingly, the number of obese patients requiring surgical procedures and anesthesia care is expected to continue growing.

Obese people represent an evident challenge in anesthesia due to their higher risk of difficult airway compared with the nonobese population.4,5 Obese patients also have a higher risk of difficult laryngoscopy due to their short, thick neck, large tongue, and significant redundant pharyngeal soft tissue, which impedes visualization of the glottis during a laryngoscopy.5 A previous study has shown that the risk of difficult tracheal intubation in obese patients is three times higher than for nonobese patients.5 However, there is still no established predictor for difficult airway in obese patients. Some studies have reported that body mass index (BMI) and oropharyngeal Mallampati score serve as predictors for difficult laryngoscopy.6–8 However, other studies refuted these correlations.9–12 Some previous studies have demonstrated that neck circumference and an abundance of pretracheal soft tissue, measured ultrasonically, may predict difficult laryngoscopy.8,11,13,14 By contrast, other studies showed no association between neck circumference, anterior neck soft tissue depth, and difficult laryngoscopy.15,16 The results of previous studies were mixed and inconclusive due to the inconsistent and incomprehensive evaluations of airway characteristics which are potentially related to difficult laryngoscopy.6–16 In addition, the predictive role of ultrasonographic airway measures for difficult laryngoscopy has been rarely explored in obese populations.13,15

We conducted a prospective observational study to explore potential risk factors for difficult laryngoscopy in obese patients undergoing bariatric surgery. In this study, ultrasound, an accessible imaging modality for anesthesiologists, was used to evaluate the tongue size and pretracheal soft tissue of obese patients. This study aimed to discover influential factors and to determine the prognostic ability of these factors for difficult laryngoscopy in obese patients.


2.1. Clinical setting and patient selection criteria

This study was reviewed and approved by the Institutional Review Board of Taipei Medical University in Taiwan (approval no. TMU-JIRB-N202002076), and registered in the international directory, (identifier: NCT04395248). Informed verbal and written consent were obtained from all participants before their inclusion within the study. All methods within this study were performed in accordance with the Helsinki Declaration and relevant regulations.

We prospectively enrolled patients undergoing laparoscopic sleeve gastrectomy at a medical center between May 2020 and August 2021. The inclusion criteria were, age 20–65 years and BMI ≥30 kg·m-2. Exclusion criteria were, known difficult airway, a history of head and neck surgery or radiation therapy, cervical spine injury or implants, severe cardiovascular disease (e.g., coronary arterial disease and aortic dissection), peripheral capillary oxygen saturation <90% in room air, pregnancy, and patient refusal (Fig. 1).

Fig. 1:
Study flow diagram. † Not mutually exclusive since patients could have more than one exclusion criterion.

2.2. Preoperative physical examination

Before surgery, all enrolled patients were examined for the classic morphometric factors associated with a difficult airway, as described in the lean population, including interincisor distance (<3 cm or not),17 mentohyoid distance (<4 cm or not),17 thyromental distance (<6.5 cm or not),5 neck movement (<80 degrees or not),18 and modified Mallampati score.19 In addition, neck circumference was measured at the thyroid cartilage.8,11,14 An upper lip bite test and a mandibular prognathism test were also conducted for each participant.20,21

2.3. Ultrasound evaluation for upper airway

Ultrasound was used to assess the upper airway of all enrolled patients before surgery, including pretracheal soft tissue depth13 and tongue size.22 Regarding pretracheal soft tissue, the distance from the skin to the anterior aspect of the trachea was measured in the central axis of the neck at three levels: vocal cords, thyroid isthmus, and suprasternal notch.13 In addition, a previous study has shown that the increased tongue volume and deposition of fat at the tongue base in obese people were highly associated with obstructive sleep apnea.23 To quantify the size of the tongue base using ultrasound, the patient was placed in a seated position, and a convex transducer (GE C1-5-RS, GE Healthcare, Chicago, IL, USA) at frequencies 1.5 to 5.0 MHz on a portable ultrasound device (LOGIQ, GE Healthcare, Chicago, IL, USA) was introduced to the skin of the neck in the submental region coronally, immediately cephalad to the body of the hyoid bone, and then in the area between the hyoid bone and the symphysis of the mandible. To obtain a standardized scanning level of the tongue base, the mucosal covering of the tongue was employed as the anterior border and the geniohyoid muscle as the posterior border, as internal landmarks. In addition, color flow Doppler was used to localize the lingual arteries on both sides of the lower lateral borders of the tongue base, where they entered the tongue base inferiorly22 (Fig. 2). The ultrasound transducer was then placed in the sagittal plane to visualize the mid-tongue. The patients were instructed to avoid tongue movements, swallowing, or talking. The maximal width and height of the tongue base and the maximal height of the mid-tongue were measured. The cross-sectional area (CSA) of the tongue base was calculated using the following formula: CSA = tongue base width × tongue base height × π × 0.25. All ultrasonographic measurements were performed twice to calculate the average value. The physical and ultrasonographic examinations were conducted by an independent attending anesthesiologist (Y.-H.T.).

Fig. 2:
Ultrasonographic measurement of tongue base and mid-tongue width and height. A, Lingual arteries were seen with color flow Doppler (dotted ellipses) on both sides of lower lateral borders of tongue base, where they entered tongue base inferiorly. Also seen were (1) mucosal covering of tongue, (2) genioglossus muscle, (3) geniohyoid muscle, and (4) mylohyoid muscle. B, Maximal width and height of tongue base. C, (1) Mucosal covering of tongue, (2) genioglossus muscle, (3) geniohyoid muscle, (4) mylohyoid muscle, and acoustic shadows reflecting (5) body of mandible, and (6) hyoid bone. D, Maximal height of mid-tongue.

2.4. Determination of difficult laryngoscopy

Upon arrival at the operating room, patients were initially placed in a ramped position and then moved into a reverse Trendelenburg position to achieve a 30-degree incline of the thorax before preoxygenation. After preoxygenation with 100% oxygen for 5 min, general anesthesia was induced with propofol 1.5–2.0 mg·kg-1 ideal body weight, fentanyl 2–3 μg·kg-1 total body weight, and rocuronium 0.8–1.0 mg·kg-1 ideal body weight. After the abolition of the eyelash reflex, rocuronium 0.8–1.0 mg·kg-1 ideal body weight was infused and immediately followed by a flush of 20 mL normal saline to hasten neuromuscular blockade.24,25 At 1 minute after rocuronium infusion, direct laryngoscopy intubation was performed. The laryngoscopy intubation was performed using a size−3 or −4 Macintosh blade (Rüsch Inc., Duluth, GA, USA) and a 7.5 or 8.0 mm styleted endotracheal tube (Unomedical, ConvaTec Inc., Deeside, Wales, UK). An attending anesthesiologist (Y.-M.W. or S.-Y.H.) performed and described the laryngoscopy intubation, while two senior nurse anesthetists were the assistants. All were blinded to the preoperative physical and ultrasound assessments.

The primary outcome was difficult laryngoscopy. The laryngoscopy view was graded according to the Cormack and Lehane classifications with external laryngeal pressure applied.26 Laryngoscopy views graded as III or IV were defined as difficult. The correct placement of the endotracheal tube was confirmed by end-tidal capnography. To further examine the parameters of difficult laryngoscopy, we also evaluated first-attempt success, overall success of intubation, time to successful intubation, need to change intubation devices, upper airway injury (bleeding, abrasion, or laceration), and dental injury (bleeding, trauma, or fracture). In addition, difficult tracheal intubation was assessed using the Intubation Difficulty Scale with seven parameters, and was defined as a summed score ≥5, as rated by the intubator and assistants.27

2.5. Statistical analysis

In a post hoc power calculation, the 17 patients with a neck circumference ≥49.1 cm had a 0.588 incidence rate of difficult laryngoscopy, while the 63 patients with a neck circumference <49.1 cm had a 0.111 incidence rate of difficult laryngoscopy in this cohort, which yields a statistical power of 0.98 to detect the event of difficult laryngoscopy, accepting a type I error of 5%.28 The Shapiro–Wilk test and the Kolmogorov–Smirnov test were used to examine the normality of the included variables. Normally distributed variables were presented as the mean ± standard deviation. Non-normally distributed data were expressed as the median with an interquartile range, minimum and maximum. The distributions of baseline patient characteristics and outcome variables were compared between patients with and without difficult laryngoscopy using chi-square tests or Fisher’s exact test for categorical variables and either t-tests or Wilcoxon rank-sum tests for continuous variables, as appropriate. Univariate logistic regression analyses were performed to evaluate the association between the included factors and difficult laryngoscopy. Significant factors from the univariate analyses were incorporated into the multivariable logistic regression model to determine the independent factors for difficult laryngoscopy. The diagnostic utility of predictive factors was estimated using area under the receiver operating characteristic curve (AUC). The optimal cutoff value for continuous variables was determined using the joint maximum sensitivity and specificity of the receiver operating characteristic curves (Youden’s index) associated with difficult laryngoscopy.29 As a sensitivity analysis, patients with a BMI <35 kg·m-2 were excluded to examine the independent factor and its diagnostic utility for difficult laryngoscopy. A two-sided significance level of p < 0.05 was used to define a statistically significant difference. All statistical analyses were conducted using Statistics Analysis System (SAS), version 9.4 (SAS Institute Inc., Cary, NC, USA).


3.1. Baseline patient characteristics

A total of 80 patients were evaluated. Among them, 17 (21.3%) developed an event of difficult laryngoscopy. Patients with difficult laryngoscopy were more likely to be older and male and to have a history of hypertension (Table 1). Among the enrolled patients, 4 (5%) were rated as having a difficult tracheal intubation.

Table 1 - Baseline patient and clinical characteristics
Difficult laryngoscopy n = 17 No difficult laryngoscopy n = 63 p
Age, year 41.6 8.9 35.3 8.8 0.0104
Sex, male 12 70.6 26 41.3 0.0317
Body mass index, linear, kg·m-2 40.9 37.8–42.5 (31.9–47.4) 40.1 35.0–45.5 (30.9–59.2) 0.9765
Body mass index, binary, kg·m-2 0.8751
<40 8 47.1 31 49.2
≥40 9 52.9 32 50.8
Waist circumference, cm 126.7 11.3 124.9 15.0 0.6454
ASA physical status 0.8751
II 8 47.1 31 49.2
III 9 52.9 32 50.8
Current cigarette smoking 9 52.9 25 39.7 0.3264
Current alcohol drinking 4 23.5 10 15.9 0.4817
Coexisting disease
Hypertension 12 70.6 15 23.8 0.0003
Diabetes mellitus 5 29.4 9 14.3 0.1620
Chronic kidney disease 1 5.9 1 1.6 0.3820
Fatty liver 16 94.1 48 76.2 0.1704
Obstructive sleep apnea 10 58.8 22 34.9 0.0742
Preoperative blood test
Hemoglobin, g·dL-1 14.7 13.5–15.5 (11.6–16.2) 14.3 13.8–15.5 (8.7–17.8) 0.7551
Creatinine, mg·dL-1 0.78 0.72–0.99 (0.54–1.25) 0.73 0.60–0.83 (0.40–1.51) 0.0766
eGFR, mL·min·1.73 m-2 93.7 90.9–119.5 (68.3–153.9) 116.9 94.2–128.1 (53.9–189.9) 0.1384
Urea nitrogen, mg·dL-1 12 9–14 (7–18) 11 10–13 (5–26) 0.6960
Sodium, mmol·L-1 140 138–142 (130–145) 138 137–139 (134–143) 0.0504
Potassium, mmol·L-1 4.0 3.6–4.1 (3.3–4.4) 3.9 3.7–4.1 (3.3–4.4) 0.7224
Alanine aminotransferase, U·L-1 40 26–57 (21––159) 31 23–39 (12–242) 0.2895
Aspartate aminotransferase, U·L-1 35 23–46 (18–142) 35 22–47 (12–305) 0.7596
SpO2 in room air, % 97 96–97 (93–98) 97 96–98 (94–100) 0.3083
Values are mean ± standard deviation, counts (percent), or median (interquartile range; minimum and maximum).
ASA = American Society of Anesthesiologists; eGFR = estimated glomerular filtration rate; SpO2 = peripheral capillary oxygen saturation.

3.2. Parameters of laryngoscopy and tracheal intubation

Among the 17 patients with difficult laryngoscopy, three were rated as a Cormack-Lehane grade IV. First-attempt success rates for intubation were 88.2% and 100.0% for those with and without difficult laryngoscopy, respectively (p = 0.0430) (Table 2). In all patients, the trachea was intubated successfully by direct laryngoscopy. Patients with difficult laryngoscopy had a higher risk of upper airway injury (3, 17.7%) compared with those without difficulty (1, 1.6%), p = 0.0286. There was no difference in time to successful intubation, need to change intubation devices, or dental injury between the two groups.

Table 2 - Parameters of laryngoscopy and tracheal intubation
Difficult laryngoscopy n = 17 No difficult laryngoscopy n = 63 p
Cormack and Lehane’s classification <0.0001
I 0 0 37 58.7
II 0 0 26 41.3
III 14 82.4 0 0
IV 3 17.7 0 0
First-attempt success of intubation 15 88.2 63 100.0 0.0430
Overall success of intubation 17 100.0 63 100.0 NA
Time to successful intubation, s 26 18–40 (13–130) 23 18–29 (11–61) 0.1229
Need to change intubation devices 0 0 0 0 NA
Upper airway injury 3 17.7 1 1.6 0.0286
Dental injury 0 0 0 0 NA
Values are counts (percent) or median (interquartile range; minimum and maximum).

3.3. Physical and ultrasound characteristics associated with difficult laryngoscopy

Patients with difficult laryngoscopy had a greater neck circumference (median 49.5 cm, IQR, 43.2–51.5 cm) compared with those without difficulty (median 43.0, IQR, 41.0–46.0 cm); the mean difference was 4.6 cm [95% confidence interval (CI), 2.2–7.1]. In addition, the CSA of the tongue base was larger in those with difficult laryngoscopy (mean 17.4 ± 5.7 cm2) compared to their counterparts (14.6 ± 3.5 cm2), mean difference 2.9 cm2 (95% CI, 0.7–5.1) (Table 3). Based on Youden’s index, the optimal cutoff neck circumference value was 49.1 cm with a 0.588 sensitivity and a 0.889 specificity. The absolute risk difference of neck circumference <49.1≥ cm was 0.477 (95% CI, 0.231–0.724) with a number needed to treat of 3. In the same way, the cutoff of the tongue base CSA was determined to be 16.4 cm2 with a sensitivity of 0.647 and a specificity of 0.698. The absolute risk difference of tongue base CSA and <16.4≥ cm2 was 0.346 (95% CI, 0.092–0.599) with a number needed to treat of 3. Table 4 shows the factors significantly associated with difficult laryngoscopy in the univariate analyses, including age, sex, hypertension, neck circumference, and CSA of the tongue base. After adjusting for these variables, neck circumference was the only independent predictive factor for difficult laryngoscopy, with an adjusted odds ratio of 1.227 (95% CI, 1.009–1.491). Supplementary Table S1 shows the results of the univariate and multivariable analyses among patients with a BMI ≥35 kg·m-2.

Table 3 - Preoperative physical and ultrasound findings
Difficult laryngoscopy n = 17 No difficult laryngoscopy n = 63 P
Physical findings
Interincisor distance <3 cm 0 0 1 1.6 >0.9999
Any loose teeth 1 5.9 3 4.8 >0.9999
Mentohyoid distance <4 cm 0 0 2 3.2 >0.9999
Thyromental distance <6.5 cm 0 0 0 0 NA
Neck movement < 80 degrees 4 23.5 7 11.1 0.2341
Neck circumference, cm 49.5 43.2–51.5 (38.5–59.5) 43.0 41.0–46.0 (34.5–56.0) 0.0020
Modified Mallampati score 0.2921
 Class I or II 10 58.8 28 44.4
 Class III or IV 7 41.2 35 55.6
Upper lip bite test 0.2341
 Class I 10 58.8 47 74.6
 Class II or III 7 41.2 16 25.4
Mandibular prognathism test 0.5157
Class A 12 70.6 50 79.4
Class B or C 5 29.4 13 20.6
Ultrasound findings
Height of tongue base, mm 45.2 40.6–47.8 (27.9–64.0) 40.7 37.1–46.0 (29.7–52.3) 0.0560
Width of tongue base, mm 47.8 41.6–53.6 (31.9–62.6) 46.3 40.7–48.6 (26.7–58.5) 0.0996
CSA of tongue base, cm2 17.4 5.7 14.6 3.5 0.0118
Maximal height of mid-tongue, mm 49.7 6.5 47.7 6.4 0.2561
Pretracheal soft tissue depth, mm
Level of vocal cords 7.7 6.5–9.2 (3.8–11.4) 7.7 5.9–10.0 (3.8–18.6) 0.9812
Level of thyroid isthmus 9.2 7.2–10.6 (6.0–21.8) 9.1 6.5–11.8 (3.8–21.0) 0.5025
Level of suprasternal notch 13.4 12.2–14.9 (9.0–26.4) 15.3 12.2–18.0 (8.8–34.9) 0.1123
Average depth 9.3 9.0–11.3 (7.3–18.2) 10.6 8.8–12.8 (6.3–22.2) 0.5604
Values are mean ± standard deviation, counts (percent), or median (interquartile range; minimum and maximum).
CSA = cross-sectional area.

Table 4 - Univariate and multivariable logistic regression analyses for difficult laryngoscopy
Univariate Multivariable
Crude OR (95% CI) p Adjusted OR (95% CI)a p
Age, year 1.079 (1.014–1.149) 0.0167 1.054 (0.972–1.143) 0.2056
Sex, male vs. female 3.415 (1.073–10.868) 0.0376 0.689 (0.099–4.789) 0.7064
Hypertension 7.678 (2.328 –25.328) 0.0008 3.264 (0.760–14.021) 0.1116
Neck circumference, cm 1.223 (1.080–1.384) 0.0015 1.227 (1.009–1.491) 0.0399
CSA of tongue base, cm2 1.002 (1.000–1.003) 0.0205 1.001 (0.999–1.003) 0.2576
CI = confidence interval; CSA = cross-sectional area; OR = odds ratio.
aAdjusted for age (linear), sex, hypertension, neck circumference (linear), and CSA of tongue base (linear).

3.4. Diagnostic utility for difficult laryngoscopy

The AUC of neck circumference ≥49.1 cm and the CSA of tongue base ≥16.4 cm2 for difficult laryngoscopy were 0.739 (95% CI, 0.612–0.865) and 0.673 (0.543–0.803), respectively. A combination of both factors offered better predictive performance than either alone with an AUC of 0.801 (0.674–0.928). In subgroup analyses, the predictive performance of neck circumference and CSA of the tongue base was better in patients with a BMI ≥40 kg·m-2 (Table 5). Supplementary Table S2 shows the diagnostic utility of neck circumference and CSA of the tongue base as dichotomous variables among patients with a BMI ≥35 kg·m-2.

Table 5 - Diagnostic utility of neck circumference and tongue base cross-sectional area for predicting difficult laryngoscopy
Neck circumference ≥49.1 cm CSA of tongue base ≥16.4 cm2 Both factors
AUC (95% CI) p AUC (95% CI) p AUC(95% CI) P
All 0.739 (0.612–0.865) 0.0002 0.673 (0.543–0.803) 0.0094 0.801 (0.674–0.928) <0.0001
Body mass index ≥40 kg·m-2 0.780 (0.618–0.941) 0.0007 0.701 (0.534–0.869) 0.0184 0.868 (0.752–0.984) <.0001
Body mass index <40 kg·m-2 0.688 (0.508–0.867) 0.0404 0.637 (0.437–0.837) 0.1785 0.742 (0.531–0.953) 0.0249
Obstructive sleep apnea 0.686 (0.503–0.870) 0.0464 0.668 (0.487–0.850) 0.0696 0.771 (0.593–948) 0.0029
No obstructive sleep apnea 0.761 (0.561–0.962) 0.0107 0.652 (0.442–0.861) 0.1563 0.781 (0.557–1.000) 0.0141
AUC = area under receiver operating characteristic curve; CI = confidence interval; CSA = cross-sectional area.


The results of this prospective observational study demonstrated that neck circumference was an independent predictive factor for difficult laryngoscopy in obese patients. None of the classic risk factors for difficult intubation described in the lean population, such as modified Mallampati score, were satisfactory in obese patients. Using the method of ultrasonography, we also discovered that CSA of the tongue base was potentially associated with difficult laryngoscopy, although this association disappeared after controlling for covariates. The predictive ability of neck circumference and tongue base size was better in patients who were morbidly obese. Considering the higher risk of difficult tracheal intubation and peri-intubation rapid desaturation in obese patients, our results provide a way of reducing unanticipated difficult airway and preventing hypoxemia during tracheal intubation in this high-risk population.

Although obese patients have a higher risk of difficult laryngoscopy, there is still no definite predictor in the current literature due to conflicting results in previous studies.6–16 Differences in the measurements and definitions of difficult airway and the evaluated airway characteristics are potentially responsible for the discrepancies in previous findings. We found that greater neck circumference was linked to difficult laryngoscopy, in line with some previous studies8,11,14 but not the study by Magalhaes et al.16 This might be explained by the significantly lower BMI and neck circumference in the study by Magalhaes et al. compared with those of our study and others.8,11,14,16 Moreover, our results suggested that neither BMI nor modified Mallampati score were correlated with difficult laryngoscopy, contrasting with some previous studies6–8 but agreeing with others.9–12 Eiamcharoenwit et al. reported that neck circumference and modified Mallampati score show limited predictive performance for difficult intubation among obese parturients.30 In our study, the optimal cutoff value for neck circumference was 49.1 cm and yielded an AUC of 0.739, with a risk difference of 0.477, which was higher than those in the previous study.30 This might be due to the smaller average neck circumference of the study by Eiamcharoenwit et al. in comparison with our study.30

Interestingly, our study found that CSA of the tongue base may be used as a potential predictor for difficult laryngoscopy in obese patients. Previous studies have shown that tongue base size is associated with the diagnosis of obstructive sleep apnea and its severity.22,23 Of note, obstructive sleep apnea has been reported to be associated with difficult intubation in obese and nonobese patients.16,31,32 Our subgroup analysis showed that the predictive performance of tongue base CSA was better in those with obstructive sleep apnea compared with others, which is compatible with previous studies.16,22,23,31,32 Our results suggested that the increased volume and fat deposition at the tongue base might impede visualization of the glottis and tracheal intubation.

The overall incidence of difficult intubation was 5% in our study, which is similar to the 3–4% reported by three previous studies12,14,30 but considerably lower than the 13% reported by Riad et al.8 This difference might be a result of the higher proportion of morbidly obese patients (i.e., BMI ≥40 kg·m-2) in the study by Riad et al.8 although some studies have refuted the association between BMI and difficult tracheal intubation among obese patients.9–12

There were some limitations to our study. First, our patient sample was relatively small and might be underpowered for some statistics. Second, some ultrasonographic parameters were not examined, such as skin to epiglottis and skin to hyoid distances.33 Third, our results may not be applicable to non-Asian populations due to the ethnic differences in morphometric and airway characteristics.34 Fourth, although the risk of difficult laryngoscopy, defined as a Cormack and Lehane’s grade III or IV, was as high as 21.3% in this study, all patients were intubated successfully by direct laryngoscopy. The predictive role of neck circumference for difficult tracheal intubation remains undetermined if video laryngoscopy serves as a standard technique to establish an airway for obese patients.35 Last, the study cohort included patients with a BMI between 30 and 35 kg·m-2, who might not have indications for laparoscopic sleeve gastrectomy.36

In conclusion, our study found that greater neck circumference was independently associated with difficult laryngoscopy in obese patients. Using ultrasound quantification of the tongue base size, this study also showed that CSA of the tongue base may serve as an index to identify high-risk patients before tracheal intubation, although this result needs further validation using datasets from patients with different characteristics (e.g., a greater BMI or an unparalyzed state). Our findings provide a way of decreasing unanticipated difficult airway in obese populations. The high risk of desaturation during tracheal intubation warrants more studies to discover novel predictors for difficult intubation in obese patients.


The authors are deeply indebted to Professor Ta-Liang Chen, Dr. Chuen-Chau Chang, and Dr. Yuan-Wen Lee of Taipei Medical University, Taipei, Taiwan for their feedback in creating the manuscript.


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Difficult airway; Difficult intubation; Neck circumference; Obstructive sleep apnea; Tongue base

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